1. Field of the Invention
The present invention relates to a variable group delay compensator, which is used for optical communication systems and optical measurement systems.
2. Description of the Related Art
Lately, a transfer system using a single wavelength cannot respond to recent optical communication in which the amount of information is excessively increased. Then, the optical communication uses a wavelength division multiplexing (WDM) system, in which light having a plurality of different wavelengths is intensity-modulated and is multiplexed to form wavelength division multiplexing light, and the formed wavelength division multiplexing light is transferred via a single optical fiber.
However, when signal light which is intensity-modulated is transferred in the optical fiber, propagation constants of the signal light are varied depending on wavelengths. Therefore, light dispersion in which propagation velocities are different depending on the wavelengths is caused in the signal light.
The signal light which is transferred in the optical fiber has a band including spectrum that is widened from a central wavelength of a channel thereof. As mentioned above, since the light dispersion is caused during the transfer of the signal light in the optical fiber, propagation velocities corresponding to spectrums in the channel are varied in a channel band and the signal light is outputted with a wave profile different from an incident wave profile.
When a transfer signal is converted into a digital signal and an optical signal which is intensity-modulated is transferred through the optical fiber, if a transfer distance is long, the propagation velocities are varied depending on the spectrums in the channel of the optical signal and the pulse width therefore becomes wide. Thus, adjacent pulses cannot be discriminated and, then, an error is easily caused. In particular, as the transfer speed of the signal light is higher, and a frequency interval between the adjacent pulses is narrower to increase a communication capacity of the optical fiber, the effect of the error is serious.
Accordingly, fast optical communication for a large capacity uses a first method in which the amount of dispersion of the optical fiber, as a transfer line, is reduced, or a second method in which a dispersion compensator having inverse characteristics of dispersion characteristics of the optical fiber is connected to the optical fiber and the dispersion characteristics of the optical fiber are compensated over a transfer wavelength band.
According to one example of the first method for reducing the amount of dispersion of the optical fiber, a dispersion shift fiber (DSF) having zero dispersion at a wavelength of 1.55 μm is used. According to one example of the second method using the dispersion compensator having the inverse characteristics of the dispersion characteristics of the optical fiber, a dispersion compensator using a dispersion compensation fiber (DCF) is adopted.
Further, another dispersion compensator (a variable group delay compensator) uses a light multiple reflector as disclosed in FIG. 13 of U.S. Pat. No. 5,930,045. Hereinbelow, a description is given of the dispersion compensator using the light multiple reflector as a conventional art of the present invention with reference to FIG. 8.
The conventional dispersion compensator (variable group delay compensator) comprises: an input/output light waveguide device 11 comprising an optical fiber 11a, a first lens 11b, a collimator lens, and a cylindrical lens; a light multiple reflector 14 including an incident plane 14a, a reflection plane 14b, and a transmission plane 14c, which are made of a glass, as basic materials, having parallel planes facing each other; a second lens 15 comprising a converging lens; and a mirror 16 in which a reflecting film is formed to have a reflectance of at least 90%. Incidentally, in the light multiple reflector 14, the reflection plane 14b is formed on one plane thereof and the transmission plane 14c is formed on an opposed plane of the reflection plane 14b. 
First, wavelength division multiplexing light, which is incident on the incident plane 14a from the input/output light waveguide device 11, strikes to the transmission plane 14c. One part of the wavelength division multiplexing light is transmitted through the transmission plane 14c and another part is reflected thereto. Next, the wavelength division multiplexing light reflected from the transmission plane 14c strikes to the reflection plane 14b but not therethrough, and is reflected to the transmission plane 14c. One part of, the light which is reflected to the reflection plane 14b and is transmitted to the transmission plane 14c, is transmitted again and another part is reflected to the reflection plane 14b. In the light multiple reflector 14, the above-mentioned reflection to the reflection plane 14b and the transmission plane 14c is repeated and a part of the wavelength division multiplexing light is outputted to the second lens 15 every strike to the transmission plane 14c. 
The wavelength division multiplexing light, which is emitted from the transmission plane 14c in varied directions depending on the wavelengths, is transmitted through the second lens 15 comprising a spherical lens. The wavelength division multiplexing light, which is transmitted through the second lens 15, is reflected at varied positions of a surface of the mirror 16 at varied angles depending on wavelength.
The wavelength division multiplexing light, which is reflected to the mirror 16, is transmitted again through the second lens 15, and is incident at varied positions with varied angles on the transmission plane 14c of the light multiple reflector 14. Thereafter, the wavelength division multiplexing light is repeatedly reflected to the reflection plane 14b and the transmission plane 14c, and is outputted from the incident plane 14a. The light outputted from the incident plane 14a is propagated through the input/output light waveguide device 11 and is outputted. That is, the wavelength division multiplexing (WDM) light is propagated in accordance with the above-mentioned order and is outputted after the group delay of the WDM light is compensated.
Consequently, according to a method for obtaining only the light having a specific wavelength belonging to the necessary degree, except for the light having a specific wavelength belonging to an order other than the necessary degree, the size of the mirror 16 is controlled as shown in FIG. 11.
For example, in FIG. 11, since the mirror 16 is formed with small size so as to limit a reflection area, the light having the wavelength λ11 is reflected only at one position on the surface of the mirror 16. Thus, there is one optical path of the wavelength λ11 but there are not plural optical paths thereof. As shown in FIG. 11, although the light of the wavelength λ11 is outputted through two optical paths from the light multiple reflector 14, one beam is reflected to the mirror 16 and then is returned to the light multiple reflector 14 and another beam does not strike to the mirror 16 and is not reflected to the mirror 16. In other words, the light of the wavelength λ11 has only one optical path. The above description can be applied to light of a wavelength λ13.
However, the structure shown in FIG. 11 has a problem in that the arrangement of the mirror 16 is difficult.
Light of the central wavelength (corresponding to the specific wavelength λ12 in FIG. 11) belonging to the order m1 must be returned to the output position of the light multiple reflector 14 so as to compensate for the wavelength dispersion with the specific wavelength λ12 as center and reduce the loss of the insertion. More specifically, mainly, angle adjustment is necessary at a reflecting point of the mirror 16 so as to return the light of the specific wavelength λ12 to the output position of the light multiple reflector 14.
In the case of limiting the size of the mirror having the above-mentioned structure, there is a problem in that optical positioning is necessary to limit the height of the mirror.